1. Field of the Invention
The present invention relates to a method of developing electrostatic latent image with a two-component developer comprising a non-magnetic toner and a magnetic carrier. In addition, the present invention also relates to an image forming apparatus and a process cartridge including the two-component developer.
2. Description of the Background
In electrophotographic image formation, an electrostatic latent image is formed on an image bearing member comprising a photoconductive material, and the electrostatic latent image is developed into a toner image with a charged toner. The toner image is then transferred onto and fixed on a recording medium. Electrophotographic developing methods are of two types: one-component developing methods and two-component developing methods. One-component developing methods use a one-component developer consisting essentially of a toner. Two-component developing methods use a two-component developer comprising a toner and a magnetic carrier which may have a resin covering layer.
Toner particles are more reliably charged in two-component developing methods than in one-component developing methods because carrier particles provide a greater area for frictionally charging toner particles. Therefore, two-component developing methods are advantageous in producing high quality images for an extended period of time. Additionally, two-component developing methods have higher toner supplying power ability that produces higher-density images. Thus, two-component developing methods are widely employed in full-color and high-speed image forming apparatuses.
In a typical two-component developing method, a linear speed (Vr) of a developing sleeve is greater than a linear speed (Vp) of an image bearing member. This causes undesirable image density variation or white blanks in the edge portions of an image in which the latent image potential is discontinuously and drastically changed.
In a case in which the image bearing member and the developing sleeve rotate in the same direction and the linear speed of the developing sleeve is greater than the linear speed of the image bearing member, developer particles (i.e., carrier particles holding toner particles thereon) on the developing sleeve are always ahead of an electrostatic latent image on the image bearing member. Therefore, when viewed from a boundary between a background portion and a solid image portion in the electrostatic latent image, the developer particles have already passed through the background portion. As a result, most of the toner particles held on the carrier particles migrates to the developing sleeve disposed on the opposite side of the background portion of the electrostatic latent image on the image bearing member due to background potential, i.e., VD (charged potential)−VB (direct current bias), leaving only a small amount of toner particles on the carrier particles which are contacting the image bearing member. The carrier particles are then given a charge opposite to that of the toner particles (hereinafter “counter charge”).
Therefore, when developer particles reach the trailing edge of the solid image after passing through the background portion with respect to the moving direction of the electrostatic latent image, toner particles cannot be quickly supplied to the image bearing member. Thus, white blanks undesirably appear in the trailing edge of the solid image. White blanks more notably appear in a halftone image that has a smaller developing potential.
When the linear speed of the developing sleeve is greater than that of the image bearing member, various problems occur such that white blanks appear in the trailing edge of image, as described above, lateral lines get thin, vertical lines get thick, sharpness of texts decreases, and carrier particles are deposited on images.
In attempting to solve the above-described problems, an effective developing method in which a magnet contained in a developing sleeve is rotated at a high speed has been proposed. In this method, developer particles are conveyed to an area where an electrostatic latent image is developed into a toner image (hereinafter the “developing area”) while being rotated. Therefore, toner particles held on carrier particles are prevented from migrating to the developing sleeve and counter charge is not likely to accumulate in the carrier particles, preventing the occurrence of the above-described problems.
However, this developing method has a drawback that toner particles are likely to fixedly adhere to carrier particles and covering layer of the carrier particles is easily abraded due to the active movement of the developer particles on the developing sleeve. Thus, the lifespan of the developer is undesirably shortened in this developing method.
On the other hand, in attempting to extend the lifespan of two-component developer, there has been a proposal to provide a low-surface-energy covering layer, comprised of a fluorine-based resin, silicone resin, or the like, on a core material of carrier.
Such a covering layer may include a conductive agent and a filler to control the resistance of the carrier and to improve the strength of the layer.
When the conductive agent or filler undesirably release from the covering layer, the resulting full-color image may be contaminated. To solve this problem, white conductive filler particles have been proposed. However, carriers using such white conductive particles do not have enough durability. Thus, in a developing method in which a magnet is rotated at a high speed, toner particles fixedly adhere to such carrier particles and the covering layer is easily abraded after a long period of use.
Because of these reasons, a developing method in which a magnet is rotated at a high speed which does not degrade developer has been demanded.